Begin by cubing the day-old bread and dicing the ham into bite-sized pieces. Shred the cheese using a box grater or a food processor, ensuring it’s fluffy and ready to melt.

In a large mixing bowl, whisk together the eggs, milk, mustard powder, salt, and pepper until well combined and frothy.

Grease a 9x13-inch baking dish with cooking spray or butter. Layer the cubed bread evenly across the bottom, followed by a generous sprinkle of diced ham. Next, add the shredded cheese in an even layer over the ham.

Carefully pour the egg mixture over the layered bread, ham, and cheese, making sure to cover everything evenly.

Let the assembled casserole sit for about 10 minutes at room temperature so the bread can absorb the liquid fully.

Preheat your oven to 350°F (175°C) while the casserole rests. Once ready, place the dish in the oven and bake for about 45 minutes, or until the top is golden brown and the eggs are set.

After baking, remove the casserole from the oven and let it cool for about 5-10 minutes.